<html xmlns="http://www.w3.org/1999/xhtml">
<head>

<bindings xmlns="http://www.mozilla.org/xbl"
          xmlns:xhtml="http://www.w3.org/1999/xhtml">
  <binding id="a">
    <content>
      <xhtml:div class="aparent">
        <xhtml:div class="a">
          <children>
            <xhtml:div class="b">
              TEXT
            </xhtml:div>
          </children>
        </xhtml:div>
      </xhtml:div>
    </content>
  </binding>
</bindings>

<style>

  @namespace xbl "http://www.mozilla.org/xbl";
  .a > .b { color: green; }
  .a > xbl|children > .b { text-decoration: underline; }
  .a .b { text-transform: lowercase; }
  .aparent > * > .b { background: yellow; }

  /* Inverse cases. */
  .a > * > xbl|children > .b { color: red !important; }

</style>

</head>

<body>
<div style="-moz-binding: url(#a);" />
</body>
</html>
